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mammothbanemammothbane
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liballoc: introduce String, Vec const-slicing
This change `const`-qualifies many methods on Vec and String, notably `as_slice`, `as_str`, `len`. These changes are made behind the unstable feature flag `const_vec_string_slice` with the following tracking issue: #129041
1 parent b7b9453 commit fba2983

8 files changed

+101
-129
lines changed

‎library/alloc/src/lib.rs

+1
Original file line numberDiff line numberDiff line change
@@ -114,6 +114,7 @@
114114
#![feature(const_option)]
115115
#![feature(const_pin)]
116116
#![feature(const_size_of_val)]
117+
#![feature(const_vec_string_slice)]
117118
#![feature(core_intrinsics)]
118119
#![feature(deprecated_suggestion)]
119120
#![feature(deref_pure_trait)]

‎library/alloc/src/raw_vec.rs

+5-5
Original file line numberDiff line numberDiff line change
@@ -280,7 +280,7 @@ impl<T, A: Allocator> RawVec<T, A> {
280280
/// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
281281
/// be careful.
282282
#[inline]
283-
pub fn ptr(&self) -> *mut T {
283+
pub const fn ptr(&self) -> *mut T {
284284
self.inner.ptr()
285285
}
286286

@@ -293,7 +293,7 @@ impl<T, A: Allocator> RawVec<T, A> {
293293
///
294294
/// This will always be `usize::MAX` if `T` is zero-sized.
295295
#[inline]
296-
pub fn capacity(&self) -> usize {
296+
pub const fn capacity(&self) -> usize {
297297
self.inner.capacity(size_of::<T>())
298298
}
299299

@@ -488,8 +488,8 @@ impl<A: Allocator> RawVecInner<A> {
488488
}
489489

490490
#[inline]
491-
fn ptr<T>(&self) -> *mut T {
492-
self.non_null::<T>().as_ptr()
491+
const fn ptr<T>(&self) -> *mut T {
492+
self.ptr.as_ptr() as _
493493
}
494494

495495
#[inline]
@@ -498,7 +498,7 @@ impl<A: Allocator> RawVecInner<A> {
498498
}
499499

500500
#[inline]
501-
fn capacity(&self, elem_size: usize) -> usize {
501+
const fn capacity(&self, elem_size: usize) -> usize {
502502
if elem_size == 0 { usize::MAX } else { self.cap.0 }
503503
}
504504

‎library/alloc/src/string.rs

+17-11
Original file line numberDiff line numberDiff line change
@@ -1057,7 +1057,8 @@ impl String {
10571057
#[inline]
10581058
#[must_use = "`self` will be dropped if the result is not used"]
10591059
#[stable(feature = "rust1", since = "1.0.0")]
1060-
pub fn into_bytes(self) -> Vec<u8> {
1060+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1061+
pub const fn into_bytes(self) -> Vec<u8> {
10611062
self.vec
10621063
}
10631064

@@ -1073,8 +1074,9 @@ impl String {
10731074
#[inline]
10741075
#[must_use]
10751076
#[stable(feature = "string_as_str", since = "1.7.0")]
1076-
pub fn as_str(&self) -> &str {
1077-
self
1077+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1078+
pub const fn as_str(&self) -> &str {
1079+
unsafe { str::from_utf8_unchecked(self.vec.as_slice()) }
10781080
}
10791081

10801082
/// Converts a `String` into a mutable string slice.
@@ -1093,7 +1095,7 @@ impl String {
10931095
#[must_use]
10941096
#[stable(feature = "string_as_str", since = "1.7.0")]
10951097
pub fn as_mut_str(&mut self) -> &mut str {
1096-
self
1098+
unsafe { str::from_utf8_unchecked_mut(&mut self.vec) }
10971099
}
10981100

10991101
/// Appends a given string slice onto the end of this `String`.
@@ -1163,7 +1165,8 @@ impl String {
11631165
#[inline]
11641166
#[must_use]
11651167
#[stable(feature = "rust1", since = "1.0.0")]
1166-
pub fn capacity(&self) -> usize {
1168+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1169+
pub const fn capacity(&self) -> usize {
11671170
self.vec.capacity()
11681171
}
11691172

@@ -1426,8 +1429,9 @@ impl String {
14261429
#[inline]
14271430
#[must_use]
14281431
#[stable(feature = "rust1", since = "1.0.0")]
1429-
pub fn as_bytes(&self) -> &[u8] {
1430-
&self.vec
1432+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1433+
pub const fn as_bytes(&self) -> &[u8] {
1434+
self.vec.as_slice()
14311435
}
14321436

14331437
/// Shortens this `String` to the specified length.
@@ -1799,8 +1803,9 @@ impl String {
17991803
#[inline]
18001804
#[must_use]
18011805
#[stable(feature = "rust1", since = "1.0.0")]
1806+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
18021807
#[rustc_confusables("length", "size")]
1803-
pub fn len(&self) -> usize {
1808+
pub const fn len(&self) -> usize {
18041809
self.vec.len()
18051810
}
18061811

@@ -1818,7 +1823,8 @@ impl String {
18181823
#[inline]
18191824
#[must_use]
18201825
#[stable(feature = "rust1", since = "1.0.0")]
1821-
pub fn is_empty(&self) -> bool {
1826+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1827+
pub const fn is_empty(&self) -> bool {
18221828
self.len() == 0
18231829
}
18241830

@@ -2559,7 +2565,7 @@ impl ops::Deref for String {
25592565

25602566
#[inline]
25612567
fn deref(&self) -> &str {
2562-
unsafe { str::from_utf8_unchecked(&self.vec) }
2568+
self.as_str()
25632569
}
25642570
}
25652571

@@ -2570,7 +2576,7 @@ unsafe impl ops::DerefPure for String {}
25702576
impl ops::DerefMut for String {
25712577
#[inline]
25722578
fn deref_mut(&mut self) -> &mut str {
2573-
unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2579+
self.as_mut_str()
25742580
}
25752581
}
25762582

‎library/alloc/src/vec/mod.rs

+14-9
Original file line numberDiff line numberDiff line change
@@ -1238,7 +1238,8 @@ impl<T, A: Allocator> Vec<T, A> {
12381238
/// ```
12391239
#[inline]
12401240
#[stable(feature = "rust1", since = "1.0.0")]
1241-
pub fn capacity(&self) -> usize {
1241+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1242+
pub const fn capacity(&self) -> usize {
12421243
self.buf.capacity()
12431244
}
12441245

@@ -1545,8 +1546,9 @@ impl<T, A: Allocator> Vec<T, A> {
15451546
/// ```
15461547
#[inline]
15471548
#[stable(feature = "vec_as_slice", since = "1.7.0")]
1548-
pub fn as_slice(&self) -> &[T] {
1549-
self
1549+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1550+
pub const fn as_slice(&self) -> &[T] {
1551+
unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
15501552
}
15511553

15521554
/// Extracts a mutable slice of the entire vector.
@@ -1563,7 +1565,7 @@ impl<T, A: Allocator> Vec<T, A> {
15631565
#[inline]
15641566
#[stable(feature = "vec_as_slice", since = "1.7.0")]
15651567
pub fn as_mut_slice(&mut self) -> &mut [T] {
1566-
self
1568+
unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
15671569
}
15681570

15691571
/// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
@@ -1618,9 +1620,10 @@ impl<T, A: Allocator> Vec<T, A> {
16181620
/// [`as_mut_ptr`]: Vec::as_mut_ptr
16191621
/// [`as_ptr`]: Vec::as_ptr
16201622
#[stable(feature = "vec_as_ptr", since = "1.37.0")]
1623+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
16211624
#[rustc_never_returns_null_ptr]
16221625
#[inline]
1623-
pub fn as_ptr(&self) -> *const T {
1626+
pub const fn as_ptr(&self) -> *const T {
16241627
// We shadow the slice method of the same name to avoid going through
16251628
// `deref`, which creates an intermediate reference.
16261629
self.buf.ptr()
@@ -2556,8 +2559,9 @@ impl<T, A: Allocator> Vec<T, A> {
25562559
/// ```
25572560
#[inline]
25582561
#[stable(feature = "rust1", since = "1.0.0")]
2562+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
25592563
#[rustc_confusables("length", "size")]
2560-
pub fn len(&self) -> usize {
2564+
pub const fn len(&self) -> usize {
25612565
self.len
25622566
}
25632567

@@ -2573,7 +2577,8 @@ impl<T, A: Allocator> Vec<T, A> {
25732577
/// assert!(!v.is_empty());
25742578
/// ```
25752579
#[stable(feature = "rust1", since = "1.0.0")]
2576-
pub fn is_empty(&self) -> bool {
2580+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
2581+
pub const fn is_empty(&self) -> bool {
25772582
self.len() == 0
25782583
}
25792584

@@ -3123,15 +3128,15 @@ impl<T, A: Allocator> ops::Deref for Vec<T, A> {
31233128

31243129
#[inline]
31253130
fn deref(&self) -> &[T] {
3126-
unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
3131+
self.as_slice()
31273132
}
31283133
}
31293134

31303135
#[stable(feature = "rust1", since = "1.0.0")]
31313136
impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
31323137
#[inline]
31333138
fn deref_mut(&mut self) -> &mut [T] {
3134-
unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
3139+
self.as_mut_slice()
31353140
}
31363141
}
31373142

‎tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir

+31-47
Original file line numberDiff line numberDiff line change
@@ -5,66 +5,48 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
55
let mut _0: &[u8];
66
scope 1 (inlined <Vec<u8> as Deref>::deref) {
77
debug self => _1;
8-
let mut _6: usize;
9-
scope 2 (inlined Vec::<u8>::as_ptr) {
8+
scope 2 (inlined Vec::<u8>::as_slice) {
109
debug self => _1;
11-
let mut _2: &alloc::raw_vec::RawVec<u8>;
12-
scope 3 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
13-
debug self => _2;
14-
let mut _3: &alloc::raw_vec::RawVecInner;
15-
scope 4 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
16-
debug self => _3;
17-
scope 5 (inlined alloc::raw_vec::RawVecInner::non_null::<u8>) {
10+
let mut _6: usize;
11+
scope 3 (inlined Vec::<u8>::as_ptr) {
12+
debug self => _1;
13+
let mut _2: &alloc::raw_vec::RawVec<u8>;
14+
scope 4 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
15+
debug self => _2;
16+
let mut _3: &alloc::raw_vec::RawVecInner;
17+
scope 5 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
1818
debug self => _3;
1919
let mut _4: std::ptr::NonNull<u8>;
20-
scope 6 (inlined Unique::<u8>::cast::<u8>) {
20+
scope 6 (inlined Unique::<u8>::as_ptr) {
2121
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
2222
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
23-
scope 7 (inlined NonNull::<u8>::cast::<u8>) {
23+
scope 7 (inlined NonNull::<u8>::as_ptr) {
2424
debug self => _4;
25-
scope 8 (inlined NonNull::<u8>::as_ptr) {
26-
debug self => _4;
27-
let mut _5: *const u8;
28-
}
29-
}
30-
}
31-
scope 9 (inlined #[track_caller] <Unique<u8> as Into<NonNull<u8>>>::into) {
32-
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
33-
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
34-
scope 10 (inlined <NonNull<u8> as From<Unique<u8>>>::from) {
35-
debug ((unique: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
36-
debug ((unique: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
37-
scope 11 (inlined Unique::<u8>::as_non_null_ptr) {
38-
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
39-
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
40-
}
25+
let mut _5: *const u8;
4126
}
4227
}
4328
}
44-
scope 12 (inlined NonNull::<u8>::as_ptr) {
45-
debug self => _4;
46-
}
47-
}
48-
}
49-
}
50-
scope 13 (inlined std::slice::from_raw_parts::<'_, u8>) {
51-
debug data => _5;
52-
debug len => _6;
53-
let _7: *const [u8];
54-
scope 14 (inlined core::ub_checks::check_language_ub) {
55-
scope 15 (inlined core::ub_checks::check_language_ub::runtime) {
5629
}
5730
}
58-
scope 16 (inlined std::mem::size_of::<u8>) {
59-
}
60-
scope 17 (inlined align_of::<u8>) {
61-
}
62-
scope 18 (inlined slice_from_raw_parts::<u8>) {
31+
scope 8 (inlined std::slice::from_raw_parts::<'_, u8>) {
6332
debug data => _5;
6433
debug len => _6;
65-
scope 19 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
66-
debug data_pointer => _5;
67-
debug metadata => _6;
34+
let _7: *const [u8];
35+
scope 9 (inlined core::ub_checks::check_language_ub) {
36+
scope 10 (inlined core::ub_checks::check_language_ub::runtime) {
37+
}
38+
}
39+
scope 11 (inlined std::mem::size_of::<u8>) {
40+
}
41+
scope 12 (inlined align_of::<u8>) {
42+
}
43+
scope 13 (inlined slice_from_raw_parts::<u8>) {
44+
debug data => _5;
45+
debug len => _6;
46+
scope 14 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
47+
debug data_pointer => _5;
48+
debug metadata => _6;
49+
}
6850
}
6951
}
7052
}
@@ -75,8 +57,10 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
7557
_2 = &((*_1).0: alloc::raw_vec::RawVec<u8>);
7658
StorageLive(_3);
7759
_3 = &(((*_1).0: alloc::raw_vec::RawVec<u8>).0: alloc::raw_vec::RawVecInner);
60+
StorageLive(_4);
7861
_4 = copy (((((*_1).0: alloc::raw_vec::RawVec<u8>).0: alloc::raw_vec::RawVecInner).0: std::ptr::Unique<u8>).0: std::ptr::NonNull<u8>);
7962
_5 = copy (_4.0: *const u8);
63+
StorageDead(_4);
8064
StorageDead(_3);
8165
StorageDead(_2);
8266
StorageLive(_6);

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